Method of producing porous metal bodies for use in the electronic industry

ABSTRACT

The production of highly porous sintered bodies useful for the manufacture of electrolytic capacitors by using as a lubricant for the base metal powder, usually tantalum, about 0.01 to about 10% by weight of an inorganic lubricant, particularly a sulfide, selenide or telluride of a metal of subgroup V or VI of the Periodic Table or nitrides of boron or silicon.

The present invention relates to the production of highly poroussintered bodies (compacts). More particularly, the invention relates tothe production of metallic anodes for the manufacture of electrolyticcapacitors by the addition of lubricants for facilitating the molding(pressing) operation and, at the same time, improving the electricalqualities of the finished capacitors.

Metal anodes for electrolytic capacitors are customarily produced bypressing the metal powder while adding an organic molding aid, such as,for example, camphor or wax, in order to conserve the molding tools and,furthermore, in order to obtain molded articles having the desired lowgreen density. The latter is absolutely required in order to insure thedesired porosity in the sintered metal body. The greater the porosity,the larger will be the inner surface of the molded article and, thereby,also the surface of the oxide layer produced by the forming, which, as adielectric, also helps to determine the capacitance of the capacitor. By"charge" is meant the product of the capacitance and the formingvoltage, according to formula L= Vf·C and

Vf= forming voltage in V C= capacitance in μF

L= charge.

The surface may be influenced by many factors such as, for example, bythe use of a highly porous powder or by low sintering temperatures or bylow molding density or even by means of a shortened sintering period.However, in practice it is not possible to change all indicatedinfluencing factors beyond certain limits since adequate mechanicalstrengths are also expected from the finished metal bodies. Furthermore,during the sintering of the metal bodies, a post-purification takesplace which depends on the sintering temperature. Therefore, neither thesintering period nor the sintering temperature can be lowered too much.If, a maximum charge per gram (μFV/g) of a sinter body is still desired,the pressing density must be decreased. Customarily this is done inaccordance with known and conventional methods, by the addition of anorganic binder which accomplishes the lubrication of the tool as well asthe binding of the powder particles with each other. At the same time,this provides also a statistically uniform distribution of the pressedmetal powders in the pressed articles. The known binders are camphor(natural or synthetic), carbon wax, nibrin wax, stearic acid,acetylcellulose, either along or in combination. Usually 2 to 10 percentby weight is added in order to obtain the desired properties for themolding step.

However, these known and practiced methods have the disadvantage ofhaving to remove the organic binder prior to the sintering process. Thisis done through heat processing between 200° and 900° C. The "dewaxingprocess" must be carried out in a vacuum due to the tendency to oxidizeof the pertinent metal powders, which results in additional expense.

If metal powders are pressed without binder addition, the molding toolswear out unusually fast. Moreover, the molded article has non-homogenousdensities. These disadvantages cannot be adequately overcome eventhrough the known use of halogenated organic solvents, liquid orgaseous, such as, for example, trichloroethylene, Freon(dichlorodifluoromethane), and the like, because the variation indensity in the pressed article remains unsatisfactory.

Observations over many years have shown that during the molding of metalpowder into anodes, without adding lubricants, approximately 50,000pressings can be expected before the tool will wear out. If, at the sameanode size, one uses, for example, trichloroethylene as a lubricant, theexpected life of the tools may be up to 100,000 pressing operations.With an addition of 2% by weight camphor and with the use of the samemetal powder, and of the same tool steel for producing the same anodemagnitude, a life expectancy of approximately 300,000 pressing processesmay be possible. This shows how important lubrication is for the moldingstep.

It must therefore be concluded from the prior art that the addition of alubricant is necessary for an economical production of molded anodes.The applied lubricants must be totally removed prior to the sinteringprocess in order to prevent the negaative influence on the sinter body.As previously described, the removal of the lubricant entails much cost.

In accordance with the present invention it was surprisingly discoveredthat when known inorganic lubricants are employed as a molding aid, theyhave a very beneficial influence upon the pressing cycle, in a mannerwhereby the tool life is considerably prolonged and the distribution ofthe metal powder in the pressed articles becomes homogeneous.Preferably, 0.01 to 10% by weight, preferably 0.02 to 5% by weight, ofan inorganic lubricant is used based on the total weight of the moldingmixture.

The green compact articles can be subjected to sintering without theremoval of the inorganic lubricant. The densities of the sintered bodiesobtained are particularly often below the green densities of the pressedbodies. Despite the fact a portion of the lubricants remains in theanodes, following the sintering process, a considerable improvement ofthe electrical properties is attained since said inorganic lubricantaffects the electrical properties of the resulting sintered body.

Particularly suitable lubricants, according to the invention, are thesulfides, selenides and tellurides of sub-groups IV, V and VI of thePeriodic Table and the nitrides of boron and silicon. Furthermore, itwas found that a portion of the lubricants can be provided by throughinorganic oxides, admixed with the lubricant, prior to the mixing of thelubricant with the valve metal powder, without diminishing the positivequalities of the lubricant. The basic metal may be tantalum and itsalloys as well as other metals of group IVB, VB and VIB of the periodicsystem and their alloys.

The following Examples will illustrate the advantage provided by thepresent invention without limiting its scope. The tests were conductedwith sintering temperatures of 1,550° to 1,850° C., sintering periodslasting from 10 to 40 minutes and the pressing densities of the greencompacts were 4 to 8 g./cm³.

In order to better judge the effect of the individual lubricantadditives, the tantalum metal powder capacitor quality used, the greendensity of the pressed anodes, the sintering time and the sintertemperatures are kept constant in the following Examples. This makes itpossible directly to deduce the effect of the additives with respect tothe valves of the charge, the residual current, the series resistance,the break-down voltage and the density of the sintered anodes.

EXAMPLE 1

Conventional tantalum metal capacitor quality (Tantalum metal powdergrade 290 produced by Hermann C. Starck Berlin, according tospecifications) was thoroughly mixed with 0.25% Mo-sulfide (Molyform 15,extra fine, made by Hermann C. Starck Berlin, average particle size 0.6to 0.8 my). This mixture was compressed on a Dorst TPA-4 S Exenterpress, into anodes with a diameter of 6.5 mm. The weight of an anode was2 gram and the green density was about 7 g/cm³. Subsequently, the anodeswere sintered for 30 minutes at 1,600° C. The sintered, finished anodeshad a density of 6.8 g/cm³. The analysis showed a residual amount of0.16% Mos₂ in the sintered anodes, which corresponds to 64% of the solidlubricant Mo-sulfide used. The electrical test of the anodes obtainedaccording to the invention produced a leakage current of 2.5 μA/g at acharge of 8,540 μFV/g, a break-down voltage of 134 V and an equivalentseries resistance of 6.4 Ohm.

If the same tantalum metal grade 290 is compressed without an additive,with the same green density and, under the same conditions, thefollowing electrical values are obtained:

leakage current 2.4 μA/g

charge 6420 μFV/g

The equivalent series resistance is 7 Ohm and the breakdown voltage is131 V.

It is noteworthy that the sinter density of the anodes, according to theinvention, is 3% below the green density and 8% below the sinter densityof the comparison anodes. The charge per gram rose by 33%. Using theanodes of the invention of Example 1, there were produced usingconventional methods dry capacitors with a solid semiconductor coating,of the type 150 μF/30 V. On reversing poles, these capacitors showed nochange in the leakage current, while capacitors of the comparison anodeswere chargeable only up to 2.5 V. in the opposite direction.

EXAMPLE 2

Tantalum metal powder capacitor quality 290 R (as in Example 1) wasintensively mixed with 0.8% Ta-sulfide and compressed on anodes of greendensity 7 g/cm³ with the same arrangement as in Example 1. After 30minutes of sintering at 1600° C., sinter anodes of a 6.9 g/cm³ densitywere obtained, with a residual content of 0.064% TaS₂ corresponding to8% of the Ta-sulfide employed. The electrical test showed that theleakage current had dropped to 1 μA/g, which represents an outstandingvalue. The capacity was still at 6700 μFV/g at an equivalent seriesresistance of 5.8 Ohm. The breakdown voltage rose to 150 V.

EXAMPLE 3

Tantalum metal powder 290 was thoroughly mixed with 0.2% boron nitride,as in Example 1, pressed to a green density of 7 g/cm³ and sintered for30 minutes, at 1600° C. Analysis showed that 55% of the employedboron-nitride was still in the sintered anodes. The sinter density ofthe anodes was determined as 6.7 g/cm³. The electrical test showed aleakage current of 1.9 μA/g, at a charge of 7500 μFV/g, a seriesresistance of 6.8 Ohm and a breakdown voltage of 128 V.

The tests were also conducted with a number of other solid lubricants,with and without an oxide addition. The essential results of these testsare compiled in Tables 1 to 4. They all show the positive influence ofthe solid lubricants, in accordance with the invention, upon theelectrical qualities of the finished, sintered anodes. In addition itshould be noted that by using the mixture of Example 1, no substantialattach on the pressing tool could be observed after 200,000 pressings,while when no lubricant was added, the pressing tools had to be replacedafter 150,000 pressings.

                                      TABLE 1                                     __________________________________________________________________________                                  without                                                       Sulfide         addition                                        __________________________________________________________________________    Lubricant     MoS.sub.2                                                                         WS.sub.2                                                                          MoS.sub.2                                                                         TaS.sub.2                                           Tantalum Metal Powder                                                                       290 290 290 290 290                                             Lubricant additive                                                                          0.25%                                                                             0.20%                                                                             3%  0.8%                                                                              without                                                                       addition                                        % Lubricant in the sin-                                                                     0.16%                                                                             0.06%                                                                             0.1%                                                                              0.06%                                                                             --                                              tered metal                                                                   Green density g/cm.sup.3                                                                    7.0 7.0 7.0 7.0 7.0                                             Sinter density g/cm.sup.3                                                                   6.8 6.8 6.9 6.9 7.4                                             Sinter Temperature ° C.                                                              1600                                                                              1600                                                                              1600                                                                              1600                                                                              1600                                            Sinter time in minutes                                                                      30  30  30  30  30                                              Charge in μFV/g                                                                          8540                                                                              8500                                                                              6800                                                                              6700                                                                              6420                                            Leakage current in μA/g                                                                  2.5 2.9 4.0 1.0 2.4                                             Equivalent series resis-                                                      tance Ω 6.4 6.3 6.8 5.8 7.0                                             % Lubricant of the the em-                                                                  64  30  3.3 7.5 --                                              ployed amount in the                                                          sintered body                                                                 __________________________________________________________________________

                  TABLE 2                                                         ______________________________________                                                     Selenide  without addition                                       ______________________________________                                        Lubricant      WSe.sub.2                                                                             TaSe.sub.2                                             Tantalum Metal Powder                                                                        290     290      290                                           % Lubricant additive                                                                         0.28%   0.28%    without addition                              % Lubricant in the sin-                                                                      0.043%  0.045%   --                                            tered metal                                                                   Green density g/cm.sup.3                                                                     7.0     7.0      7.0                                           Sinter density g/cm.sup.3                                                                    7.1     7.4      7.4                                           Sinter Temperature ° C.                                                               1600    1600     1600                                          Sinter time in minutes                                                                       30      30       30                                            Charge in μFV/g                                                                           7300    6800     6420                                          Leakage current in μA/g                                                                   2.6     1.8      2.4                                           Equivalent series resis-                                                                     6.9     6.9      7.0                                           tance Ω                                                                 % Lubricant of the em-                                                                       15.4    16.1                                                   ployed amount in the                                                          sintered body                                                                 ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                                                   without                                                         Nitride       addition                                           ______________________________________                                        Lubricant      BN      BN      Si.sub.3 N.sub.4                               Tantalum Metal Powder                                                                        290     290     290   290                                      % Lubricant additive                                                                         0.2%    8%      0.13% without                                                                       addition                                 % Lubricant in the sin-                                                                      0.11%   2.16%   0.06%                                          tered metal                                                                   Green density g/cm.sup.3                                                                     7.0     7.0     7.0   7.0                                      Sinter density g/cm.sup.3                                                                    6.7     5.3     7.2   7.4                                      Sinter Temperature ° C.                                                               1600    1600    1600  1600                                     Sinter time in minutes                                                                       30      30      30    30                                       Charge in μFV/g                                                                           7500    6580    7200  6420                                     Leakage current in μA/g                                                                   1.9     1.7     1.0   2.4                                      Equivalent series resis-                                                                     6.8     6.0     7.1   7.0                                      tance Ω                                                                 % Lubricant of the em-                                                                       55      27      46    --                                       ployed amount in the                                                          sintered body                                                                 ______________________________________                                    

                                      TABLE 4                                     __________________________________________________________________________                                                without                                        Mixtures                       addition                          __________________________________________________________________________    Lubricant    MoS.sub.2 +  HfO.sub.2                                                                BN + NbSe.sub.2                                                                      BN + Si.sub.3 N.sub.4 +  MoO.sub.3                Tantalum Metal Powder                                                                      290     290    290             290                               % Lubricant additive                                                                       0.2% + 0.1%                                                                           0.1% + 0.3%                                                                          100 ppm + 100 ppm + 300 ppm                                                                   --                                % Lubricant in the sin-                                                                    0.16 + 0.04                                                                           0.04 + 0.028                                                                         70 ppm + 54 ppm + 120 ppm                                                                     --                                tered metal                                                                   Green density g/cm.sup.3                                                                   7.0     7.0    7.0             7.0                               Sinter density g/cm.sup.3                                                                  6.8     6.8    6.6             7.4                               Sinter Temperature ° C.                                                             1600    1600   1600            1600                              Sinter time in minutes                                                                     30      30     30              30                                Charge in μFV/g                                                                         7180    7030   7600            6420                              Leakage current in μA/g                                                                 2.0     2.5    2.3             2.4                               Equivalent series resis-                                                                   6.5     6.6    6.4             7.0                               tance Ω                                                                 % Lubricant of the em-                                                                     67      17     49              --                                ployed amount in the sin-                                                     tered body                                                                    __________________________________________________________________________

What I claim is:
 1. In a method of producing a porous sintered metalbody which comprises molding under pressure metal power to form a moldedbody and sintering said molded body, said metal being selected from thegroup consisting of groups IVB, VB, and VIB of the Periodic Table, whichcomprises prior to the molding step admixing with the metal powder about0.01 to about 10% by weight of at least one inorganic lubricant selectedfrom sulfides, selenides and tellurides of a member of subgroups V andVI of the Periodic Table, whereby the sintered metal body still contains2% to 70% by weight of the inorganic lubricant.
 2. The method accordingto claim 1, wherein a mixture of different inorganic lubricants is used.3. A method according to claim 1, wherein there is also admixed with themetal powder an oxide selected from the group consisting of a member ofsubgroupd IV, V and VI of the Periodic Table.
 4. The method according toclaim 1, wherein the metal powder comprises tantalum or a tantalumalloy.
 5. The method according to claim 1, wherein the metal powder ismixed with 0.02 to 5% by weight of the inorganic lubricant.
 6. Themethod according to claim 1, wherein there is included in the admixturean oxide selected from the group consisting of a member of subgroups IV,V and VI of the Periodic Table.
 7. A sintered molded body produced bythe method of claim
 1. 8. A sintered molded body according to claim 7molded into the shape of an anode for an electrolytic capacitor.
 9. Amixture for forming molded sintered bodies which comprises a powderedbase metal selected from the group consisting of groups IVB, VB and VIBof the Periodic Table and at least one inorganic lubricant selected fromsulfides, selenides and tellurides of a member of subgroups V and VI ofthe Periodic Table, said lubricant being present in an amount of about0.01 to about 10% by weight.
 10. A green compact produced by moldingunder pressure a mixture according to claim
 9. 11. A mixture accordingto claim 9, in which the base metal is tantalum.
 12. A mixture accordingto claim 9, which includes an oxide selected from the group consistingof a member of groups of subgroups IV, V and VI of the Periodic Table.